> I wondered why other delivery companies use commercial vehicles instead of NGDVs or LLVs, and the answer is that commercial vehicles are more capable while being significantly cheaper.

Commercial vehicles in fleets are leased - they last anywhere from 2-5 years then they're pushed off to the used vehicle market. After 10-15 years they're so done that they'll end up on the scrap yard or shipped off to Africa or Asia. On top of that they're designed to be aerodynamic at highway speeds because they'll spend a lot of their life time on such streets and speeds, whereas these postal trucks will spend most of their time moving like snails.

These USPS tanks in contrast are expected to last 30, 40 years like their predecessors, racking up insane mileage... and while no one cares if some underpaid gig worker runs over some child because the child happened to be in the dead spot of his vehicle, if the same happens to USPS the government itself is the target for a nice juicy lawsuit. So it makes sense for a custom order at that scale and expected life time, and it also makes sense to stray from what the market has to offer because the requirements are different.

Oh, and commercial delivery doesn't have to take care about its workers. They'll get burned out with their backs and joints ruined, but the companies don't have to pay a dime. In contrast, USPS is a government agency and has a massive financial incentive to keep healthcare costs low.